The present invention relates to planar array antennas, and particularly to the slotted waveguide planar array antennas.
The performance of planar arrays comprising longitudinal (shunt) slot radiators in the broad wall of conventional rectangular waveguides has a number of limitations: The scan range in the plane perpendicular to the waveguide axis (E-plane) is restricted because of the relatively large width of the waveguide which translates into a wide inter-element spacing in this plane. This wide spacing also hampers sidelobe control. The purpose of sidelobe control would have been best served by an arrangement of columns of collinear slots with narrow spacing between the columns; however this arrangement is not possible in conventional rectangular waveguides because longitudinal slots must be arranged in staggered configuration. Moreover, polarization is limited to the plane perpendicular to the waveguide axis.
Transversal (series) slots, on the other hand, while potentially providing orthogonal polarization, are not used in conventional arays because of the difficulty associated with their incorporation into a serial waveguide array.
The ridged waveguide may have a much narrower cross section than the rectangular waveguide, and as such it holds promise for constructing slot arrays with narrower inter-element spacing in the E-plane, thus providing a solution to the scan and sidelobe limitations of conventional rectangular waveguides. However, symmetrical ridge waveguides are limited in the area over which longitudinal slots can be cut; therefore the dynamic range is limited. In addition, polarization is still limited to the E-plane.
The idea of asymmetric ridge waveguides is described in H. Shnitkin and J. Green, "Asymmetric Ridge Waveguide Collinear Slot Array Antenna", U.S. Pat. No. 4,638,323, Jan. 20, 1987, and the paper J. Green, B. Shnitkin and Paul J. Bertalan, "Asymmetric Ridge Waveguide Radiating Element for a Scanned Planar Array", IEEE Trans. on Antennas and propagation Vol. 38, No. 8. pp. 1161-1165, August 1990. In order to increase the dynamic range of the slots, the ridge is constructed in an asymmetric manner, thereby allowing a wider region on one of its sides for placing longitudinal slots, but includes alternating side chambers on opposite sides of the ridge alternating between high and low level. Longitudinal slots are fed out of phase every one-half a waveguide wavelength (.lambda.g); therefore they must be placed at opposing sides of the broad walls of conventional or symmetrical ridge waveguides.
In the array antenna described in U.S. Pat. No. 4,638,323, ridge asymmetry is obtained by including, on opposite sides of the ridge, side chambers alternating between high and low levels. The above patent and the publication relating thereto also disclose the idea of providing a meandering ridge to produce ridge asymmetry. However, the structures described in that patent and publication are difficult to construct mechanically; moreover, they may generate high order modes at the bends of the ridges.
Slotted waveguide array antennas of this type are also described in U.S. Pat. Nos. 3,189,908, 4,658,261, 4,873,528, 3,193,830, 3,183,511, 4,513,291, 4,821,044, 4,554,550 and 4,554,551.